Electronic security systems are known in which a resonant circuit is electromagnetically interrogated, the presence of such resonant circuit within a predetermined area under protection being indicative of an alarm condition. Such systems are employed, for example, for pilferage control in retail stores, libraries and the like wherein the resonant circuit is affixed to items being protected, such that unauthorized removal of an item from a protected area will occasion an alarm. A preferred system is the subject of U.S. Pat. Nos. 3,810,147 and 3,863,244 of the same inventor as herein in which a resonant tag circuit is operative at two frequencies, a first frequency for sensing tag presence and a second frequency for deactivation of the circuit by altering or destroying its resonant properties at the first or detection frequency. Deactivation of the tag circuit is accomplished by applying an electromagnetic field to the circuit of a strength sufficient to fuse a conductive link therein to alter the resonance characteristics of the circuit at its detection frequency by disconnection of a portion of the circuit.
In the resonant tag circuits of the aforesaid patents, the fusible link can be placed such that upon destruction of the link the tag becomes wholly inoperative or alternatively at a position within the tag circuit such that upon destruction of the link, the resonant properties of the tag at the detection frequency only are destroyed by disconnection of part of the overall circuit. As a further alternative, the link can be so placed that its destruction can give rise to a third resonant frequency different from both the detection and destruction frequencies of the tag circuit. It is the object of this invention to provide planar resonant tag circuits of the type shown in the aforesaid patents but constructed to include an electrically alterable inductor such that the circuit can be wholly inoperative until electrically activated or which can be electrically altered by variation of circuit inductance to provide different resonant characteristics.